Can you see any reason why contraction is possible in some case (eg. My dog is cuddly = My dog 's cuddly) but not in other (However cuddly my dog is, I can't bring it to lectures)
Good question. With no real oblique cases to speak of, English depends heavily on voice inflection. The 'is' in your second example must be said in full and emphasised to assist in the meaning.
In your first example, the contracted 'is' plays a much less significant role and can be spoken so quickly, it can be represented as with 's.